The following relates to the magnetic resonance imaging (MRI) arts. It particularly relates to a short bore horizontal magnet for an MRI scanner, and will be described with particular reference thereto. However, the following also relates to other MRI scanner magnets such as long bore magnets, open magnets, and vertical magnets, and to magnets of various types for applications both inside and outside the magnetic resonance imaging arts.
In a typical MRI scanner, a cylindrical bore electromagnet is arranged to receive a subject such as a patient, or any item which exhibits magnetic resonance properties, within the magnet bore. For medical imaging, the electromagnet is arranged with the cylindrical axis oriented horizontally to readily accommodate a patient arranged in a prone or supine position on a horizontal patient support. However, MRI scanners that employ electromagnets of other geometries such as xe2x80x9copenxe2x80x9d magnets are also known.
In the past, long-bore cylindrical electromagnets with a plurality of axially spaced annular windings have been employed to achieve a large, highly uniform magnetic field oriented along the cylindrical axis. However, such magnets are large and inhibit patient access. Additionally, in medical imaging it has been found that claustrophobic or nervous patients are often intimidated by being placed in a long-bore electromagnet. Hence, there is a demand for shorter bore magnets for MRI scanners.
However, as the magnet bore is shortened, magnetic field uniformity can suffer. In particular, an imaging volume within the magnet bore which has a uniform enough magnetic field for imaging is reduced as the magnet bore is shortened. There is also a demand for high magnetic fields, which exasperates the problem of producing a large uniform field region in a compact magnet because of increased conductor requirements and larger coils. A large main magnetic field can be desirable since the magnetic resonance signal is proportional to field strength. This signal advantage can be used in a variety of ways such as to obtain high spatial resolution, rapid data acquisition rates, and other imaging benefits.
To a certain extent, magnetic field nonuniformities can be corrected using steel shims. Specifically, by adding selected, variable amounts of steel at selected locations in the magnetic field produced by the magnet, the field can be shaped to improve field uniformity. Such shimming is typically performed to compensate for manufacturing variations. That is, the shims are added during calibration to correct for manufacturing imperfections, and are typically not part of the magnet design. Shim corrections are effective for magnetic field variations that are characterized as lower order harmonic field components, but are less effective in correcting or modifying higher order magnetic field components due to a need for substantial amounts of steel to effect higher order corrections.
To further improve field uniformity and strength, electromagnets have been designed in conjunction with a steel structure, assembly, array or the like which interacts with the magnet field to define a larger imaging volume. The steel structure is incorporated into the magnet design, rather than being used to correct manufacturing imperfections.
However, a problem arises in magnets that incorporate a field shaping arrangement of ferromagnetic material. Specifically, the steel structure must be very precisely aligned within the magnet to achieve optimal field uniformity. Relative misalignment of as little as one millimeter (about 0.1% for a 1-meter magnet bore) can produce substantial magnetic field nonuniformities. Misalignment of the steel structure in the axial direction of the magnet is particularly problematic.
The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.
According to one aspect, a method is provided for aligning a magnetic field-modifying structure in a magnet bore defined by a main magnet of a magnetic resonance imaging scanner. The magnetic field-modifying structure is inserted into the magnet bore. Subsequent to the inserting, a value is measured of one of: (i) an odd harmonic component of a field cooperatively produced by the main magnet and the magnetic field-modifying structure, and (ii) a force exerted on the magnetic field-modifying structure by a field produced by the main magnet. A position of the magnetic field-modifying structure in the magnet bore is adjusted to minimize the measured value.
According to another aspect, an apparatus is disclosed for aligning a magnetic field-modifying structure in a magnet bore defined by a main magnet of a magnetic resonance imaging scanner. A field measuring means is provided for measuring a value of one of: (i) an odd harmonic component of a field cooperatively produced by the main magnet and the magnetic field-modifying structure, and (ii) a force exerted on the magnetic field-modifying structure by a field produced by the main magnet. A means is provided for indicating a position adjustment to the magnetic field-modifying structure in the magnet bore to minimize the measured value.
According to yet another aspect, a magnetic resonance imaging apparatus is disclosed, including a main magnet that defines a generally cylindrical magnet bore. A magnetic field-modifying structure is configured to modify at least one even harmonic component of a magnetic field produced by the main magnet. The magnetic field-modifying structure is aligned in the magnet bore by minimizing at least one odd harmonic component of the magnetic field.
One advantage resides in providing very precise alignment of a ferromagnetic structure, another electromagnet, or other magnetic structure, assembly, array or the like within an MRI scanner magnet bore.
Another advantage resides in enhanced magnetic field uniformity and homogeneity.
Another advantage resides in enhanced safety and speed of the alignment process.
Yet another advantage resides in aligning a steel structure with respect to a magnetic field component that is closely associated with the magnetic field component which the steel structure is intended to modify.
Numerous additional advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments.